Ion lens system for mass spectrometers and method of operation

ABSTRACT

An ion lens system which may be utilized with a mass spectrometer for varying the width of an ion beam entering or leaving the analysis region of the spectrometer, and method of operation thereof. The lens system includes a plurality of apertured ion lenses which may be positioned between an ionization chamber and the analysis region of the spectrometer, or alternatively, may be positioned between the analysis region of the spectrometer and the collector. By varying the potentials applied to the ion lenses, the effective widths of the apertures may be altered in order to vary the width of the ion beam.

United States Patent 1191 Halliday et al.

[ ION LENS SYSTEM FOR MASS SPECTROMETERS AND METHOD OF OPERATION Inventors: John S. Halliday; Sydney Evans, both of Sale, England Associated Electrical Industries Limited, London, England Filed: Nov. 4, 1970 Appl. No.: 87,020

Assignee:

[30] Foreign Application Priority Data Nov. 7, 1969 Great Britain 54646/69 [52] US. Cl. 250/419 ME, 250/4l.9 D, 250/49.5 P [51] Int. Cl. H01] 39/34 [58] Field of Search"; .Q. 250/4l.9 G, 41.9 D, 250/419 ME, 49.5 P

[56] References Cited UNITED STATES PATENTS 3,622,781. ll/l97l Liebl 250/419 1 1] 3,745,343 14 1 July 10,1973

3,487,208 12/1969 Futrelletal, ..25o/41.9 3,585,384 6/1971 Castaingetal. ..250/49.5

Primary Examiner-William F. Lindquist Att0rney-Watts, Hoffman, Fisher & Heinke 57 ABSTRACT An ion lens system which may be utilized with a mass spectrometer for varying the width of an ion beam entering or leaving the analysis region of the spectrometer, and method of operation thereof. The lens system includes a plurality of apertured ion lenses which may be positioned between an ionization chamber and the analysis region of the spectrometer, or alternatively, may be'positioned between the analysis region of the spectrometer and the collector. By varying the potentials applied to the ion lenses, the effective widths of the apertures may be altered in order to vary the width of the ion beam.

20 Claims, 6 Drawing Figures l Electrostatic 5 -\XL=IUJ:=- :ZITLI:YQ' Ana\yser Source m 1 I v42 2 .l 1! ,l I: '1'

39 Al 1 TH U mm, Collectorm p Analyser atented July 10, 1973 3 Sheets-Sheet l Elecfroshafic Anadyser Magnehc Analyser Collecror Fig.1

INVENTORS Fig.2

Patented July 10, 1973 3 Sheets-Sheet 2 INVENTORS JOHN S. HALL/DAY BY SYDNEY EVANS mw j/ 42M ,4 TTO/QNEYS Patented July 10, 1973 I I 3,745,343

I5 Sheets-Sneet i3 gm: r24

SUPPLY BUFFER F Fl LI ER 42 43 44 v 45 H I GH VOLTAGE UNI T5 i i L INVENTORS F' g 6 JOHN 5. HALL/DAY BY SYDNEY EVANS ATTORNEYS ION LENS SYSTEM FOR MASS SPECTROMETERS AND METHOD OF OPERATION It is well known in mass spectrometers for the width of the ion beam to be defined by means of mechanical slits, both on entering and leaving the analysis region. These slits may be of fixed dimensions, in which case the resolving power of the instrument can be changed only by dismantling the ion source and collector systems to replace the plates carrying the slits.

More usually in high performance instruments both the slits at the source and collector ends of the analysis region are continuously adjustable from outside the vacuum enclosure. The mechanism required to provide this adjustment usually includes a micrometer screw, a bellows through the vacuum wall, and numerous levers, pivots, springs and sliding components within the vacuum system. Considerable precision is required in the manufacture of such a mechanism if accurate and reproducible characteristics are needed. A typical range of adjustments required is from a maximum slit width of say thou (i.e. 0.005 inches) down to a minimum of l/lOth thou, the adjustment being continuously variable with an effective slit length of about 200 thou. In this example the slit is ideally straight and smooth to within l/ 100th thou and the adjustment is required to maintain the slit edges parallel to within 1/ 100th thou over the slit length.

Apart from the difficulties inherent in manufacture,

mechanically adjustable slits have a number of other disadvantages in use. The mechanism involved can give rise to gas traps and adsorbed gas can be released sporadically by the rubbing together of surfaces..Jerky movement can also result from lubrication problems in high vacuum, particularly in the presence of high sample concentrations with consequent surface contamination and under those conditions, physical blocking of the slit can occur due to sample build up on the slit edges.

One object of the present invention is to provide a construction of mass spectrometer in which the width of ion beam entering and/or leading the analysis region may be adjusted without the difficulties discussed above of using mechanically adjustable slits.

In a mass spectrometer according to the present invention, an ion zoom lens system is associated with an opening (for example a slit) through which is arranged to pass the ion beam of the spectrometer, this lens system being arranged to vary the efiective width of said opening.

Said opening may be a slit defining the width of the ion beam entering the analysis region of the spectrometer or alternatively it may be a slit defining the width of ion beam leaving the analysis region that is passed to an ion collector. In a preferred arrangement, separate source and collector slits are provided each with an associated ion zoom lens system so that the width of ion beam entering the analysis region and the width of ion beam leaving that region may be separately adjusted.

According to a feature of the present invention, a mass spectrometer has two ion lenses, which are of variable magnification and which together form an ion zoom lens system, disposed between an analysis region of the spectrometer and a source or collector slit, the

arrangement being such that the effective width of the slit through which the ion beam passes during use of the spectrometer may be varied by electrically changing the magnification of said lenses.

Preferably each of said ion lenses is formed by a plurality of apertured elements through which the ion beam is arranged to pass and there is provided means to supply variable potential to at least one element of each of these lenses to control the magnification thereof.

One example of a mass spectrometer in accordance with the present invention will now be described with reference to the accompanying drawings in which i FIG. 1 shows the complete mass spectrometer diagrammatically,

FIG. 2 is an explanatory drawing of the ion zoom lens system,

FIG. 3 shows the construction of the ion zoom lens system in more detail,

FIGS. 4 and 5 show enlarged views, partly in section, of part of the lens system of FIG. 3, FIG. 4 being a cross-section at the line IV IV in FIG. 5, and

FIG. 6 shows an electric circuit.

Referring first to FIG. I, the mass spectrometer comprises an ion source 1 including an ionization chamber (not shown) containing an ion accelerating electrode, an electrostatic anaylser 2, a magnetic analyser 3, and an ion collector 4 in known manner. (Although not shown in the drawing, the collector 4 is preferably connected to an electron multiplier and output signals from the multiplier are passed through an amplifier to a suitable read-out device.) Two slits 5 and 6 are re spectively associated with the ion source I and the collector 4, the ion source slit 5 limiting the width of the ion beam entering the analysis region of the spectrometer and the collector slit 6 limiting the width of ion beam passed to the collector 4. During use the magnetic field to which the ion beam is subjected in the magnetic analyser 3 is scanned through a range of values and during such a scan ions of different mass reach the collector 4. As so far described the mass spectrometer is quite conventional.

The slit 5 is of fixed width and for the purpose of controlling the widthof ion beam entering the analysis region there is provided an ion zoom lens system 7 between the slit 5 and the electrostatic analyser 2. The

lens system 7 consists of two ion lenses 8 and 9 which will subsequently be described in more detail.

The operation of the ion zoom lens system 7 will now be considered with reference to FIG. 2 of the accompanying drawings. As shown in this figure, the lens 8 is arranged to afl'ect the ion beam 12 so as to produce an intermediate image of the slit 5 in a plane 10 between that lens and the lens 9 while the lens 9 produces a final image of the slit 5 in a plane 11 on the opposite side of that lens, this latter plane being the object plane of the electrostatic analyser 2 (FIG. 1). In respect of the ion beam 12 the lenses 8 and 9 satisfy the usual lens formula and it can easily be shown that the overall magnification m of the lens system 7 is given by the following expression:

m im/(m s) where u, the distance between the source ion slit 5 and the lens 8,

v the distance between the lens 8 and the intermediate image plane 10,

a the distance between the plane 10 and the lens 9 and v the distance between the lens 9 and the final object plane 11.

Since the lenses 8 and 9 are fixed relative to the ion slit 5 and the rest of the mass spectrometer, it follows that u and v, are constant and that v u is also constant. However by suitably varying the focal length of the lenses 8 and 9 v and u may be varied so as to vary the overall magnification of the lens system 7.

By way of example, if u v l and a v 0.2 (so that v a 1.2), an overall magnification of unity is obtained. If however the lenses 8 and 9 are modified so that v 0.2 and a l, the overall magnification is then 1/25.

The ion zoom lens system 7 may be used in practice to effect de-magnification of the ion source slit 5 from 1:1 to more than 100:1 so that if the ion source slit has a width of 5 thou, the width of the ion beam at the object plane '11 may be varied continuously between 5 thou and U20 thou. In other words the effective width of the ion source slit may be varied between these limits.

The preferred construction of the ion zoom lens system 7 will now be described with reference to FIGS. 3, 4 and 5 of the accompanying drawings. The lens system 7 is contained within a generally cylindrical metal housing 13 which is secured by way of vacuum seals between the ion source assembly 1 (which constitutes the ion source of FIG. 1) and a coupling unit 14 which is connected to the electrostatic analyser 2 (FIG. 1 (It i may be mentioned here that the form of mass spectrometer herein described is a modification of a commercially available mass spectrometer in which the lens system 7 is not provided and the housing 13 is replaced by a housing which is somewhat shorter in length and to which the assembly 1 and the unit 14 are connected.)

The lens 8 .is formed by three apertured metal plates l5, l6 and 17 with their apertures 18 and 19 (see FIGS. 4 and 5) each having their major dimension in the plane of FIGS. 3 and 5. By means of three bolts 20, the plates 15 and 17 are clamped so as to abut respectively projections 21 from the flange 22 of a metal support member 23 and the flange 24 of another metal support member 25, tubular metal spacers 26 being provided between the plates 15 and 17. The plates 15 and 17 are maintained at earth potential as will subsequently be described.

The plate 16 is supported by three studs 28 to lie midway between the plates 15 and 17 and parallel thereto. Each stud 28 also carries quartz spacers 29 and 30 and a tube 31 of ceramic insulating material so that the plate 16 is electrically insulated from the plates 15 and 17. The end of one of the studs 28 remote from the plate 16 is connected to a spring contact 50. During use, the plate 16 is maintained at a high potential supplied over a cable 32 which passes through the housing 13 by way of a high voltage seal 33, the conductor 34 pressing against the spring contact 50.

The lens 9 is of essentially the same construction as the lens 8 and will accordingly not be described in detail. The member 51 is however secured to the unit 14 by means of bolts (not shown) so as to provide an electrical connection to maintain the plates 35 and 36 at earth potential (and also the plates 15 and 17 of the lens 7 as previously mentioned). The apertured plate 37 is maintained at a high potential supplied over a cable 38.

The slit 5 (FIG. 1) is provided by a plate 5' that is secured to the flange 27 of the member 23, the slit which is not shown in FIG. 3 lying in the plane of that figure.

It will be appreciated that the focal lengths of the lenses 8 and 9 may be changed to vary the overall magnification of the lens system 7 merely by altering the voltages supplied over the cables 32 and 38.

Reverting now to FIG. 1 of the accompanying drawings, the collector slit 6 has an associated ion zoom lens system 39 which is essentially the same as the lens system 7 described above. It will be appreciated that the mass spectrometer operates so that ions emerging from the magnetic analyser 3 effectively form an image (hereinafter termed the analyser image) of the source ion slit 5 shortly after leaving that analyser and the object plane of the lens system 39 is arranged to coincide with the plane of this image. The lens system 39 serves to produce a reduced image of the slit 6 in its object plane. Looked at another way, the lens system 39 results in a magnified image of the analyser image being produced in the plane of the collector slit 6, this magnified image being of substantially the same width or wider than the slit 6 so that ions in all or part of the analyser image are passed to the collector 4 (depending upon the particular operating conditions). As in the case of the lens system 7, it is possible to adjust the effective width of the collector slit 6 by varying the magnification of the lens system 39.

During use of the mass spectrometer described above,'the lens system 7 associated with the ion source slit 5 is utilised to control the effective width of the source ion beam while the lens system 39 associated with the collector slit 6 is utilised to control the ion transmission to the collector 4 to obtain optimum sensitivity for a given resolving power.

With this arrangement, rapid variation of resolutio may be obtained merely by altering the appropriate potentials supplied to the lens systems 7 and 39. Thus, when the magnetic field of the magnetic analyser 3 is scanned through a range of values (as aforesaid) for the purpose of detecting ions of different mass, very high resolution may be used over only a small selected part of the total mass range being investigated while the remainder of the scan may be carried out at a lower resolution and consequently higher sensitivity.

The potentials'applied to the lenses 8 and 9 of the lens system 7, for example, to efiect a given demagnification of the ion source slit 5 are dependent on the energy of the ion beam. In order to compensate automatically for changes in the ion source potential, it is convenient for the lens potentials to be dependent upon the ion source potential. A suitable circuit for effecting this is shown in FIG. 6. In this circuit, a small fraction of the voltage supplied by the power supply 40 to the ion source 1 (FIG. 1) is passed to a buffer amplifier 41 which in turn supplies to four potentiometers 42 to 45 a voltage proportional to the ion source potential. High voltage units 46 to 49 are connected respectively to the tapping points of the potentiometers 42 to 45 and these units thus supply to the lens systems 7 and 39 potentials that are dependent upon the ion source potential. During use of the mass spectrometer, the magnification of the lens system 7 may therefore be changed merely by movement of the tapping points of the .potentiometers 42 and 43 while the lens system 39 may similarly be changed by means of the potentiometers 44 and 45.

Preferably the arrangement described may be modified so that there is only a single control for each of the lens system 7 and 39. Thus, for the lens system 7, the potentiometer 43 may be replaced by a function generator which responds to the voltage supplied by the potentiometer and which supplies the appropriate voltage to the unit 47 for correct operation of the lens system 7 at all settings of the potentiometer 42.

Instead of the potentials supplied to the lens systems 7 and 39 being continuously adjustable, as provided for by the circuit of FIG. 6, they may be selected from a plurality of predetermined values. In this case the resolving power of the spectrometer is only adjustable in known steps.

In an alternative arrangement, the mass spectrometer may be arranged automatically to control its resolving power. This may be done by causing the spectrometer to scan a single ion mass (of a sample introduced by way of said ion source) and to pass back to the lens controls an electric signal that is a measure of the width (i.e. time duration) of the resulting peak measured at the collector. Having set the resolving power in this way, the ratio of the magnifications of the two lens systems is then automatically set to give the optimum transmission conditions (i.e. automatically to adjust for maximum peak height at the selected peak width),

Although in the embodiment of the invention described herein the slit 5, for example, is of fixed width, it is to be understood that the invention is not restricted to mass spectrometers having slits of fixed physical width. In practice-the overall range of the effective width of the slit 5, for example, may be increased if the slit 5 is adjustable or is arranged to be replaced by an alternative slit of different width.

Having thus described our invention, we claim:

1. In a mass spectrometer an ionization source for .ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, analyzer means disposed to apply a deflection field to a said beam of ions, and detection means positioned to receive at least a portion of the ions in said beam of ions in order to analyze said ions, wherein the improvement comprises:

an ion lens system disposed between said ion exit slit and the entire analyzer means and including;

first and a second electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said ion exit slit and the entire analyzer means to form a focused image of said ion exit slit in a focal plane between said lens means and said analyzer means;

control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of a said image of said slit in a said focal plane while maintaining the image focus in the plane whereby the lens means function in the manner of an adjustable slit.

2. An apparatus as defined in claim 1 wherein said first electrostatic lens means includes first, second, and third apertured members positioned in spaced relationship with respect to each other, said first and third apertured members being connected to ground potential.

3. An apparatus as defined in claim 1 wherein each of said first and second electrostatic lens means comprises first, second and third apertured members positioned in spaced relationship with respect to each 5 other, each of said first and third apertured members being coupled to a constant source of potential and the second apertured members of said first and second electrostatic lens means being coupled to variable sources of potential.

4. An apparatus as defined in claim 1 including a second ion lens system disposed between the entire analyzer means and said detection means and including;

first and second electrostaitc lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said ion exit slit and said detection means to thereby form an image of said ion exit slit in a second plane between said lens means and said detection means;

control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of a said image of said ion exit slit in a said second plane.

5. A method of varying the width of an ion beam in a mass spectrometer including an analyzer section, and an ion source, comprising the steps of:

ionizing asample to be analyzed to thereby form ions;

directing the ions into a beam;

passing the beam of ions through a first and second electrostatic lens means to form a focused image of a source slit in a focal plane between the lenses and the entire analyzer section; and,

electrically varying the focal lengths of the first and second electrostatic lens means in coordinated manner so as to vary the size of said image in said focal plane while maintaining the focus of the image in said plane. I

6. A method of varying the width of an ion beam as set forth in claim 5 including the steps of:

applying first and second potential signals respectively to a firstand a second apertured member in said first electrostatic lens means; applying third and fourth potential signals respectively to a first and a second apertured member in said second electrostatic lens means; and, varying the value of both of said first and third potential signals in order to. vary the width of the ion beam by changing the so focal lengths of both the first and second electrostatic lenses while maintaining the focal length of the beam substantially constant.

7. In a mass spectrometer including an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit to pass a beam of ions from the ionization source, analyzer means, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said analyzer means, and an ion collector slit disposed between said analyzer means and said detection means wherein the improvement comprises:

an ion lens system disposed between one of said ion slits and the entire analyzer means and including; first and second electrostatic lens means each having an aperture therein for the passage of a said beam of ions; said first electrostatic lens means disposed such that this lens means provides a first image in a cross over plane situated between said first electrostatic lens means and one side of said second electrostatic lens means, and said second electrostatic lens means disposed such that this lens means provides a second image in a focal plane which is positioned on the opposite side of said second electrostatic lens means; and,

control means for varying the focal length of both said first and second lens means in a manner such that the location of the cross over plane is shifted longitudinally of the beam while the image plane of the second image is at a constant position to thereby vary the magnification of said ion lens system whereby to cause the lens means to function in the manner of an adjustable slit.

8. An apparatus as defined in claim 7 wherein said one ion slit is said ion exit slit of said ionization source.

9. An apparatus as defined in claim 8 including a second ion lens system disposed between said analyzer means and said detection means and including:

third and fourth lens means each having an aperture therein for the passage of a said beam of ions and for forming a third image in a third plane between said second lens system and said detection means; and,

control means for varying the focal lengths of said third and fourth lens means in a manner such that the plane of the third image is at a constant position to thereby vary the magnification of said ion lens system.

10. An apparatus as defined in claim 7 wherein said one ion slit is said ion collector slit.

1 1. In a double focusing mass spectrometer an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, double focusing analyzer means including an electrostatic analyzer disposed to apply an electrostatic deflection field toa said beam of ions and a magnetic analyzer disposed to apply a magnetid deflection field to a said beam of ions, detection means positioned to receive at. least a portion of the ions in said beam of ions after passage through said double focusing analyzer means, and an ion collector slit disposed between said double focusing analyzer means and said detector means wherein the improvement comprises:

- an ion lens system disposed between one of said ion slits and said double focusing analyzer means and including; first and second electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between one of said ion slits and said double focusing analyzer means to thereby form an image in an image plane;

control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of said image in said image plane while maintaining the overall focal length of said lens system substantially constant whereby the lens system functions in the manner of an adjustable slit.

12. An apparatus as defined in claim 11 wherein said one ion slit is said ion exit slit of said ionization source.

13. An apparatus as defined in claim 12 including a second ion lens system disposed between said analyzer means and said detection means and including:

third and fourth electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said third and fourth electrostatic lens means disposed between said analyzer means and said detection means to thereby form a second image in a second plane between said third and fourth lens means and said detection means;

control means for varying the focal lengths of said third and fourth electrostaticlens means in a manner to vary the width of said second image in said second plane while said second image plane remains at a constant position. 14. An apparatus as defined in claim 11 wherein said one ion slit is said ion collector slit.

15. In a double focusing mass spectrometer including an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit to pass a beam of ions from the ionization source, double focusing analyzer means including an electrostatic analyzer disposed to apply an electrostatic deflection field to a said beam of ions and a magnetic analyzer disposed to apply a magnetic deflection field to a said beam of ions, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said double focusing analyzer means, and an ion collector slit disposed between said double focusing analyzer means and said detector means wherein the improvement comprises:

an ion lens system disposed between one of said ion slits and said double focusing analyzer means and including; 7

first and second electrostatic lens means each having an aperture therein for the passage of a beam of ions; said first electrostatic lens means disposed such that this lens means provides a first image in across over plane situated between said first electrostatic lens means and said second electrostatic lens means, and said second electrostatic lens means disposed such that this lens means provides a second image in an image plane; and,

control means for varying the focal length of both said first and second electrostatic lens means while maintaining the total focal length of both of said electrostatic lens means substantially constant to thereby vary the magnification of said ion lens system in the image plane whereby the lens system functions in the manner of an adjustable slit.

16. An apparatus as defined in claim 15 wherein said one ion slit is said ion exit of said ionization source.

17. An apparatus as defined in claim 16 including a second ion lens system disposed between said analyzer means and said detection means and including:

third and fourth electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said analyzer means and said detection means to thereby form an image in a second image plane between said third and fourth lens means and said detection means;

control means for varying the focal lengths of said third and fourth electrostatic lens means in a manner to vary the width of said image in said second image plane.

18. An apparatus as defined in claim 15 wherein said one ion slit is said ion collector slit.

19. In a mass spectrometer an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, analyzer means, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said analyzer means, and an ion collector slit disposed between said analyzer means and said detector means wherein the improvement comprises:

a. a first ion lens system disposed between said ion exit slit and the analyzer means and including:

i. first and second lens means each having an aperture therein for the passage of a said beam of ions and for forming an image in an image plane;

ii. control means for varying the focal lengths of said first and second lens means in a manner to vary the width of said image in said image plane while maintaining the focal length of said beam substantially constant;

b. a second ion lens system disposed between said analyzer means and said detection means and includmg:

i. third and fourth lens means each having an apersaid third and fourth lens means in a manner to varyv the width of said second image in said second image plane while maintaining the focal length of said beam substantially constant.

analyzer means through first and second electrostatic lens means to form a focused image in the plane of said slit; and,

electrically varying the focal lengths of the first and second electrostatic lens means in co-ordinated manner so as to vary the size of said image insaid slit plane while maintaining the image focused in said plane.

i s =0: 4: as 

1. In a mass spectrometer an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, analyzer means disposed to apply a deflection field to a said beam of ions, and detection means positioned to receive at least a portion of the ions in said beam of ions in order to analyze said ions, wherein the improvement comprises: an ion lens system disposed between said ion exit slit and the entire analyzer means and including; first and a second electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said ion exit slit and the entire analyzer means to form a focused image of said ion exit slit in a focal plane between said lens means and said analyzer means; control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of a said image of said slit in a said focal plane while maintaining the image focus in the plane whereby the lens means function in the manner of an adjustable slit.
 2. An apparatus as defined in claim 1 wherein said first electrostatic lens means includes first, second, and third apertured members positioned in spaced relationship with respect to each other, said first and third apertured members being connected to ground potential.
 3. An apparatus as defined in claim 1 wherein each of said first and second electrostatic lens means comprises first, second and third apertured members positioned in spaced relationship with respect to each other, each of said first and third apertured members being coupled to a constant source of potential and the second apertured members of said first and second electrostatic lens means being coupled to variable sources of potential.
 4. An apparatus as defined in claim 1 including a second ion lens system disposed between the entire analyzer means and said detection means and including; first and second electrostaitc lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said ion exit slit and said detection means to thereby form an image of said ion exit slit in a second plane between said lens means and said detection means; control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of a said image of said ion exit slit in a said second plane.
 5. A method of varying the width of an ion beam in a mass spectrometer including an analyzer section, and an ion source, comprising the steps of: ionizing a sample to be analyzed to thereby form ions; directing the ions into a beam; passing the beam of ions through a first and second electrostatic lens means to form a focused image of a source slit in a focal plane between the lenses and the entire anAlyzer section; and, electrically varying the focal lengths of the first and second electrostatic lens means in coordinated manner so as to vary the size of said image in said focal plane while maintaining the focus of the image in said plane.
 6. A method of varying the width of an ion beam as set forth in claim 5 including the steps of: applying first and second potential signals respectively to a first and a second apertured member in said first electrostatic lens means; applying third and fourth potential signals respectively to a first and a second apertured member in said second electrostatic lens means; and, varying the value of both of said first and third potential signals in order to vary the width of the ion beam by changing the focal lengths of both the first and second electrostatic lenses while maintaining the focal length of the beam substantially constant.
 7. In a mass spectrometer including an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit to pass a beam of ions from the ionization source, analyzer means, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said analyzer means, and an ion collector slit disposed between said analyzer means and said detection means wherein the improvement comprises: an ion lens system disposed between one of said ion slits and the entire analyzer means and including; first and second electrostatic lens means each having an aperture therein for the passage of a said beam of ions; said first electrostatic lens means disposed such that this lens means provides a first image in a cross over plane situated between said first electrostatic lens means and one side of said second electrostatic lens means, and said second electrostatic lens means disposed such that this lens means provides a second image in a focal plane which is positioned on the opposite side of said second electrostatic lens means; and, control means for varying the focal length of both said first and second lens means in a manner such that the location of the cross over plane is shifted longitudinally of the beam while the image plane of the second image is at a constant position to thereby vary the magnification of said ion lens system whereby to cause the lens means to function in the manner of an adjustable slit.
 8. An apparatus as defined in claim 7 wherein said one ion slit is said ion exit slit of said ionization source.
 9. An apparatus as defined in claim 8 including a second ion lens system disposed between said analyzer means and said detection means and including: third and fourth lens means each having an aperture therein for the passage of a said beam of ions and for forming a third image in a third plane between said second lens system and said detection means; and, control means for varying the focal lengths of said third and fourth lens means in a manner such that the plane of the third image is at a constant position to thereby vary the magnification of said ion lens system.
 10. An apparatus as defined in claim 7 wherein said one ion slit is said ion collector slit.
 11. In a double focusing mass spectrometer an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, double focusing analyzer means including an electrostatic analyzer disposed to apply an electrostatic deflection field to a said beam of ions and a magnetic analyzer disposed to apply a magnetic deflection field to a said beam of ions, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said double focusing analyzer means, and an ion collector slit disposed between said double focusing analyzer means and said detector means wherein the improvement comprises: an ion lens system disposed between one of said ion slits and said double focusing analyzer means and including; first and second elEctrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between one of said ion slits and said double focusing analyzer means to thereby form an image in an image plane; control means for varying the focal lengths of said first and second electrostatic lens means in a manner to vary the width of said image in said image plane while maintaining the overall focal length of said lens system substantially constant whereby the lens system functions in the manner of an adjustable slit.
 12. An apparatus as defined in claim 11 wherein said one ion slit is said ion exit slit of said ionization source.
 13. An apparatus as defined in claim 12 including a second ion lens system disposed between said analyzer means and said detection means and including: third and fourth electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said third and fourth electrostatic lens means disposed between said analyzer means and said detection means to thereby form a second image in a second plane between said third and fourth lens means and said detection means; control means for varying the focal lengths of said third and fourth electrostatic lens means in a manner to vary the width of said second image in said second plane while said second image plane remains at a constant position.
 14. An apparatus as defined in claim 11 wherein said one ion slit is said ion collector slit.
 15. In a double focusing mass spectrometer including an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit to pass a beam of ions from the ionization source, double focusing analyzer means including an electrostatic analyzer disposed to apply an electrostatic deflection field to a said beam of ions and a magnetic analyzer disposed to apply a magnetic deflection field to a said beam of ions, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said double focusing analyzer means, and an ion collector slit disposed between said double focusing analyzer means and said detector means wherein the improvement comprises: an ion lens system disposed between one of said ion slits and said double focusing analyzer means and including; first and second electrostatic lens means each having an aperture therein for the passage of a beam of ions; said first electrostatic lens means disposed such that this lens means provides a first image in a cross over plane situated between said first electrostatic lens means and said second electrostatic lens means, and said second electrostatic lens means disposed such that this lens means provides a second image in an image plane; and, control means for varying the focal length of both said first and second electrostatic lens means while maintaining the total focal length of both of said electrostatic lens means substantially constant to thereby vary the magnification of said ion lens system in the image plane whereby the lens system functions in the manner of an adjustable slit.
 16. An apparatus as defined in claim 15 wherein said one ion slit is said ion exit of said ionization source.
 17. An apparatus as defined in claim 16 including a second ion lens system disposed between said analyzer means and said detection means and including: third and fourth electrostatic lens means each having an aperture therein for the passage of a said beam of ions, said first and second electrostatic lens means disposed between said analyzer means and said detection means to thereby form an image in a second image plane between said third and fourth lens means and said detection means; control means for varying the focal lengths of said third and fourth electrostatic lens means in a manner to vary the width of said image in said second image plane.
 18. An apparatus as defined in claim 15 wherein said one ion slit is said ion colleCtor slit.
 19. In a mass spectrometer an ionization source for ionizing a sample to be analyzed, said source having an ion exit slit for the passage of a beam of ions, analyzer means, detection means positioned to receive at least a portion of the ions in said beam of ions after passage through said analyzer means, and an ion collector slit disposed between said analyzer means and said detector means wherein the improvement comprises: a. a first ion lens system disposed between said ion exit slit and the analyzer means and including: i. first and second lens means each having an aperture therein for the passage of a said beam of ions and for forming an image in an image plane; ii. control means for varying the focal lengths of said first and second lens means in a manner to vary the width of said image in said image plane while maintaining the focal length of said beam substantially constant; b. a second ion lens system disposed between said analyzer means and said detection means and including: i. third and fourth lens means each having an aperture therein for the passage of a said beam of ions and for forming a second image in a second image plane between said third and fourth lens means and said detection means; and, ii. control means for varying the focal lengths of said third and fourth lens means in a manner to vary the width of said second image in said second image plane while maintaining the focal length of said beam substantially constant.
 20. A method of varying the width of an ion beam in a mass spectrometer which includes an analyzer section, an ion source to supply a beam of ions to said analyzer section, a detector positioned to receive at least a portion of the ions in said beam of ions that emerge from said analyzer section after passage therethrough, and a slit disposed between the analyzer section and the detector, comprising the steps of: passing the ions of said beam that emerge from said analyzer means through first and second electrostatic lens means to form a focused image in the plane of said slit; and, electrically varying the focal lengths of the first and second electrostatic lens means in co-ordinated manner so as to vary the size of said image in said slit plane while maintaining the image focused in said plane. 